Research on a high dynamic range (HDR) imaging based on multi-frame exposure combination has been extremely mature. For example, in a consumer market, an iPhone® is integrated with an HDR photographing function. Android® reference platforms of some chips also have a relatively simple HDR imaging function.
Currently, both surveillance cameras for a professional application and mobile phone cameras in the consumer market use a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensor to capture an optical signal in a scene and transform the optical signal into raw data of a digital signal (that is, Raw data). After being processed by a series of digital signal processing technologies (e.g., Image Signal Processing (ISP)), the raw signal is transformed into a final video signal that is suitable for human eyes (generally in red green blue (RGB) format or luminance and chrominance (YUV) format).
The Raw data is single-channel data, which is generally formed by multiple adjacent pixels that are arranged repeatedly and are in different colors, and each pixel location has only one value. However, data after color interpolation processing in the ISP increases to three-channel data, that is, each pixel location has three values.
Most HDR processing technologies are applied to three-channel data after the ISP processing, for example, Microsoft® (Special Interest Group on Graphics (SIGGRAPH) 2003 HDR video) proposes a post processing method for correcting multiple exposure frames. First, a camera response curve is calibrated by using the multiple exposure frames; and then an HDR image is combined according to the camera response curve; finally, a dynamic range is compressed by using a tone mapping method to improve local contrast.
Disadvantages of the foregoing method are as follows. Triple amount of raw data needs to be processed, and further, such methods as local tone mapping or contrast enhancement need to be used to improve the local contrast. High complexity is involved. In addition, a gamma correction in the ISP processing greatly affects brightness of an image. Such method depends on the camera response curve and the tone mapping, and if the method is simply applied to the Raw data for direct processing, it may cause that colors and contrast of the image are seriously distorted.